Method for production of hydrogen storage alloy for use in alkaline storage batteries

ABSTRACT

A method of producing a hydrogen storage alloy, for use in alkaline storage batteries, includes two steps. A first step involves preparing alloy particles having a CaCu 5 -type crystal structure and the compositional formula MmNi x Co y Mn z M 1−z , wherein M represents at least one element selected from the group consisting of aluminum (Al) and copper (Cu), 3.0≦x≦5.2, 0≦y≦1.2, 0.1≦z≦0.9, and 4.4≦x+y+z≦5.4. A second step involves immersing the alloy particles in an acid treating solution containing a cobalt compound and a copper compound, each in the amount of 0.1 to 5.0% by weight based on the weight of the alloy particles, and an organic additive to remove oxide films from and to reductively deposit cobalt and copper on a surface of each alloy particle to form a surface region surrounding a bulk region and having a graded composition. When the sum in percentage of numbers of cobalt (Co) atoms and copper (Cu) atoms present in the surface region is given by a and that in the bulk region by b, the relationship a/b≧1.3 is satisfied.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a Divisional of copending U.S. applicationSer. No. 09/720,615, filed Dec. 26, 2000.

TECHNICAL FIELD

[0002] The present invention relates to a hydrogen storage alloy usefulfor a negative electrode of an alkaline storage battery and especiallyto a method for production thereof.

BACKGROUND ART

[0003] A nickel-hydrogen storage battery has been recently noted as anew alkaline storage battery because of its high capacity, at leasttwice as high as that of nickel-cadmium batteries, and its environmentalfriendly nature. With the spread of portable instruments, thisnickel-hydrogen storage battery is expected to further increase itsperformance.

[0004] A hydrogen storage alloy, when incorporated in a negativeelectrode of the nickel-hydrogen storage battery, generally undergoesspontaneous oxidation to form an oxide layer on its surface.Accordingly, a hydrogen storage alloy electrode fabricated from such ahydrogen storage alloy, when used as a negative electrode of thenickel-hydrogen storage battery, presents a problem of low initialbattery capacity that is attributed to the initial low activity ofhydrogen storage alloy.

[0005] A method has been recently proposed, for example, in JapanesePatent Laying-Open No. Hei 5-225975, which immerses a hydrogen storagealloy in an acid solution, such as a hydrochloric acid solution, toremove an oxide layer formed on its surface.

[0006] This method contemplates to remove an oxide layer from a surfaceof the hydrogen storage alloy by immersing it in the acid solution.However, nickel and cobalt hardly elute into the acid solution so thatactive sites as of metallic nickel (Ni) and cobalt (Co) appear on thehydrogen storage alloy surface.

[0007] Upon removal of the oxide layer by the above-described method,the active sites as of metallic nickel and cobalt appear on the hydrogenstorage alloy surface, so that the initial discharge capacity isincreased. The reduction of electrical contact resistance between alloyparticles also results to increase the high-rate discharge capacity to aslight degree. However, the electrical contact resistance between alloyparticles is still too high to achieve a marked improvement in high-ratedischarge capacity. Also, the method is still insufficient to preventthe buildup of pressure in the battery and improve a charge-dischargecycle life of the battery.

[0008] It is an object of the present invention to provide a hydrogenstorage alloy which, when fabricated into an electrode for alkalinestorage batteries, can provide an excellent charge-discharge cycle lifeperformance, prevent the buildup of battery's internal pressure duringovercharge and improve high-rate discharge characteristics, and also toprovide a method for production thereof.

DISCLOSURE OF THE INVENTION

[0009] The hydrogen storage alloy of the present invention, for use inalkaline storage batteries, has a CaCu₅-type crystal structure and isrepresented by the compositional formula MmNi_(x)Co_(y)Mn_(z)M_(1−z),(wherein M represents at least one element selected from the groupconsisting of aluminum (Al) and copper (Cu); x is a nickel (Ni)stoichiometry and satisfies 3.0≦x≦5.2; y is a cobalt (Co) stoichiometryand satisfies 0≦y≦1.2; z is a manganese (Mn) stoichiometry and satisfies0.1≦z≦0.9; and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4).Characteristically, the hydrogen storage alloy has a bulk regionsurrounded by a surface region. The bulk region has a CaCu₅-type crystalstructure and a substantially uniform composition while the surfaceregion has a graded composition. When the sum of percentages by numberof cobalt (Co) atoms and copper (Cu) atoms present in the surface regionis given by a and the sum of percentages by number of cobalt (Co) atomsand copper (Cu) atoms present in the bulk region is given by b, therelationship a/b≧1.3 is satisfied.

[0010] In the present invention, the percentage by number of cobalt (Co)or copper (Cu) atoms present in the bulk or surface region may bereferred to in terms of atomic %.

[0011] As stated above, in the hydrogen storage alloy of the presentinvention, the bulk region is a region that has a CaCu₅-type crystalstructure and a substantially uniform composition. The surface region isa region that surrounds the bulk region and has a graded composition.This surface region is the region of the hydrogen storage alloy particlethat undergoes a change in composition when it is immersed in an acidtreating solution according to the production method of the presentinvention which will be described later. By this immersion treatment,any oxides present on an alloy particle surface is removed while cobaltand copper are reductively deposited. As a result, the surface region isallowed to contain the increased amounts of cobalt and copper atomscompared to the bulk region. As-described above, when the sum ofpercentages by number of cobalt atoms and copper atoms present in thesurface region is given by a and the sum of percentages by number ofcobalt atoms and copper atoms present in the bulk region is given by b,the relationship a/b≧1.3 is satisfied. If a/b falls below 1.3, it maybecome difficult to obtain the effect of the present invention thatimproves a charge-discharge cycle life performance by lowering thecontact resistance between hydrogen storage alloy particles to therebyincrease a discharge capacity. It may also become hard to obtain thefurther effect of the present invention that not only prevents thebuild-up of battery's internal pressure during overcharge but alsoimproves high-rate discharge characteristics.

[0012] In the present invention, the region that encompasses a surfaceand its vicinity and has a graded composition is defined as the surfaceregion, as contrary to the bulk region that has a substantially uniformcomposition. The surface region is generally observed to have acomposition gradient such that the percentages by number of cobalt andcopper atoms present therein increase toward the surface. Accordingly,the sum of percentages by number of those atoms present in the surfaceregion, a, is determined by an average value in the surface region.Since the sum of percentages by number of those atoms measured at anintermediate depth of the surface region generally comes close to theaverage value in the surface region, the measured value may be taken asthe sum of percentages by number of cobalt and copper atoms present inthe surface region.

[0013] When the hydrogen storage alloy having the above-specifiedcrystal structure and compositional formula is used for negativeelectrode material of an alkaline storage battery, the negativeelectrode shows the suppressed corrosion in an electrolyte to absorb theincreased amount of hydrogen. This is the reason why the presentinvention uses the hydrogen storage alloy having such crystal structureand composition.

[0014] The surface region generally extends inwardly from an alloyparticle surface to the depth of 80 nm.

[0015] The hydrogen storage alloy electrode of the present invention,for use in alkaline storage batteries, is obtained by loading thehydrogen storage alloy of the present invention in an electricallyconductive substrate such as a punching metal.

[0016] The method for producing a hydrogen storage alloy of the presentinvention comprises a first step wherein alloy particles are preparedhaving a CaCu₅-type crystal structure and represented by thecompositional formula MmNi_(x)Co_(y)M_(z)M_(1−z) (wherein M representsat least one element selected from the group consisting of aluminum (Al)and copper (Cu); x is a nickel (Ni) stoichiometry and satisfies3.0≦x≦5.2; y is a cobalt (Co) stoichiometry and satisfies 0≦y≦1.2; z isa manganese (Mn) stoichiometry and satisfies 0.1≦z≦0.9; and the sum ofx, y and z satisfies 4.4≦x+y+z≦5.4), and a second step wherein the alloyparticles are immersed in an acid treating solution containing a cobaltcompound and a copper compound, each in the amount of 0.1-5.0% by weightbased on the weight of alloy particles, to remove oxide layers on alloyparticle surfaces and deposit cobalt and copper reductively so thatsurface regions are formed at alloy particle surfaces.

[0017] In the first step, alloy particles are prepared having thecrystal structure and compositional formula as specified above. Thosealloy particles are generally prepared by adding specific types ofmetals to a Misch metal consisting of a mixture of rare-earth metals.The technique used to prepare such alloy particles is not particularlyspecified. They may be prepared by casting the metal mixture into aningot and then subdividing the ingot, or alternatively, by utilizing agas atomizing or roll quenching technique. In view of sinterability ofresulting alloy particles, the use of gas atomizing technique ispreferred.

[0018] In the second step, the alloy particles are immersed in an acidtreating solution containing a cobalt compound and a copper compoundeach in the amount of 0.1-5.0% by weight, based on the weight of alloyparticles. This treatment not only removes oxide layers from alloyparticle surfaces but also allows reductive deposition of cobalt andcopper, resulting in the formation of surface regions at the alloyparticle surfaces. Examples of acids useful for preparation of the acidtreating solution include hydrochloric acid, nitric acid and phosphoricacid.

[0019] Examples of cobalt and copper compounds for addition to the acidtreating solution include cobalt chloride (CoCl₂), cobalt hydroxide(Co(OH)₂), copper chloride (CuCl₂) and copper hydroxide (Cu(OH)₂).

[0020] The cobalt and copper compounds are incorporated in the acidtreating solution in concentrations of 0.1-5.0% by weight, respectively,for the reasons which follow. The respective loadings thereof, if exceed5.0% by weight, cause cobalt and copper to be deposited in excessivelylarge amounts that result in the increased tendency of alloy particlesto be oxidized and, if fall below 0.1% by weight, cause cobalt andcopper to be deposited in small amounts that result in the difficulty tosatisfy the relationship a/b≧1.3. More preferably, the cobalt and coppercompounds are incorporated in the acid treating solution inconcentrations of 0.3-5.0% by weight, respectively.

[0021] Preferably, the acid treating solution is initially maintained ata pH in the range of 0.7-2.0. The pH of below 0.7 in some cases causesrapid oxidation of alloy particles that may dissolve even an interior ofthe hydrogen storage alloy. On the other hand, the pH of greater than2.0 may result in the insufficient removal of oxide layers.

[0022] The acid treating solution may further contain at least oneorganic additive selected from the group consisting of 2,2′-bipyridyl,diethyldithio carbamate, 2-mercaptobenzothiazole and metanilic yellow.Such organic additives, if present, promote reductive deposition ofcobalt and copper. The organic additive may preferably be incorporatedin the amount of 5-50 ppm and contributes to further improvement ofbattery characteristics if kept within the specific range.

[0023] The method of producing a hydrogen storage alloy for use inalkaline storage batteries, in accordance with the present invention, ischaracterized as including the step of loading the hydrogen storagealloy of the present invention in an electrically conductive substratesuch as a punching metal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic sectional view illustrating hydrogen storagealloy particles of the present invention.

[0025]FIG. 2 is a diagrammatic sectional view illustrating an alkalinestorage battery.

BEST MODE FOR CARRYING OUT THE INVENTION

[0026] Examples and Comparative Examples of the present invention aredescribed below in detail. However, the present invention is not limitedto the following Examples and can be practiced by adding suitablemodifications within the range that does not depart from the gist of thepresent invention.

[0027]FIG. 1 is a schematic sectional view, illustrating hydrogenstorage alloy particles of the present invention. The hydrogen storagealloy particles 1 are configured such that each includes a bulk region 3and a surface region 2 located toward a surface to surround the bulkregion 3. Accordingly, the bulk region 3 is enclosed by the surfaceregion 2. As shown in FIG. 1, the depth of the surface region 2 is notnecessarily consistent and may be varied from location to location. Ingeneral, the surface region 2 extends from its surface mostly to thedepth of 80 nm.

[0028] In the present invention, when the sum of percentages by number(atomic %) of cobalt atoms 4 and copper atoms 5 present in the surfaceregion 2 is given by a and the sum of percentages by number (atomic %)of cobalt atoms 4′ and copper atoms 5′ present in the bulk region 3 isgiven by b, the relationship a/b≧1.3 is satisfied.

[0029] The bulk region 3 is a region which has a CaCu₅-type crystalstructure and a substantially uniform composition, while the surfaceregion 2 is a region which has been formed by the aforestated treatmentwith the acid solution and has a composition different from that of thebulk region 3. The surface region 2 generally has a composition gradientsuch that cobalt atoms 4 and copper atoms 5 are more concentrated towarda surface. Accordingly, the percentages by weight (atomic %) of cobaltatoms 4 and copper atoms 5 present in the surface region 2 is given byaverage values. Generally, the percentages by number of cobalt andcopper atoms present in the vicinity of an intermediate depth of thesurface region 2 determine those average values. Preferably, thepercentages by number of cobalt atoms 4 and copper atoms 5 aredetermined from measurements at several locations within the surfaceregion 2.

[0030] (Experiment 1)

[0031] In this Experiment 1, various hydrogen storage alloys, for use inalkaline storage batteries, were determined for the sum, a, ofpercentages by number (atomic %) of cobalt and copper atoms present inthe surface region, and the sum, b, of percentages by number (atomic %)of cobalt and copper atoms present in the bulk region located interiorof each hydrogen storage alloy particle. After calculation of a/b, itsrelation to battery characteristics was investigated.

[0032] Descriptions follow in the order of preparation of alloyparticles, preparation of samples, assembly of alkaline storagebatteries and detailed analysis.

[0033] (Preparation of MmNi_(3.1)Co_(0.9)Mn_(0.6)M_(0.4) AlloyParticles)

[0034] Mm (Misch metal, a mixture of rare-earth metals, consisting, byweight, of 25% La, 50% Ce, 7% Pr and 18% Nd), and Ni, Co, Mn and Al(99.9% pure metal used for each), as starting materials, were mixed in amolar ratio of 1.0:3.1:0.9:0.6:0.4, allowed to melt in an electric arcfurnace under argon atmosphere and then cooled naturally to produce aningot represented by the compositional formulaMmNi_(3.1)Co_(0.9)Mn_(0.6)M_(0.4). This ingot was mechanicallysubdivided in the air in such a controlled fashion as to provide alloyparticles having an average particle size of 80 μm.

[0035] (Samples A1-A6 and Sample X)

[0036] A cobalt compound, cobalt chloride (CoCl₂), was added to anaqueous solution of hydrochloric acid in the amount of 0.1% by weight,based on the weight of alloy particles to be treated. A copper compound,copper chloride (CuCl₂), was also added to the aqueous solution ofhydrochloric acid in the amount as indicated in Table 1, based on theweight of alloy particles to be treated. The above procedures result inan acid treating solution having a pH of 1.0. The alloy particles wereimmersed in the above-prepared acid treating solution maintained at 25°C., agitated for 30 minutes, and then suction filtered. Afterfiltration, the alloy particles were washed with water and dried. As aresult, hydrogen storage alloy samples A1-A6 were obtained.

[0037] For comparative purposes, an aqueous solution of hydrochloricacid having a pH of 1.0, exclusive of cobalt chloride and copperchloride, was prepared. The alloy particles were immersed in thisaqueous solution of hydrochloric acid maintained at 25° C. and agitatedfor 30 minutes. After suction filtration, the alloy particles werewashed with water and dried. As a result, an hydrogen storage alloysample X was obtained.

[0038] (Assembly of Batteries)

[0039] 100 parts by weight of each of the above-obtained hydrogenstorage alloy samples and 20 parts by weight of a 5 wt. % solution ofPEO (polyethylene oxide) in water, as a binder, were mixed to prepare apaste. This paste was applied (loaded) onto opposite sides of anelectrically conductive substrate comprised of a nickel-plated punchingmetal which was subsequently dried at room temperature and cut to aspecified size. As a result, hydrogen storage alloy electrodes suitablefor use in alkaline storage batteries were fabricated.

[0040] An AA-size positive limited alkaline storage battery (batterycapacity of 1,000 mAh) was assembled by using each hydrogen storagealloy electrode as a negative electrode. The alkaline storage batteryalso used a conventionally known sintered type nickel plate for apositive electrode, an alkali-resistant nonwoven fabric for a separatorand a 30 wt. % aqueous solution of potassium hydroxide for a liquidelectrolyte.

[0041]FIG. 2 is a diagrammatic sectional view of an alkaline storagebattery construction as assembled. The alkaline storage battery includesa positive electrode 11, a negative electrode 12, a separator 13, apositive lead 14, a negative lead 15, a positive external terminal 16, anegative case 17 and a sealing cover 18.

[0042] The positive electrode 11 and negative electrode 12 areaccommodated within the negative case 17 in a spirally woundconfiguration with the separator 13 between them. The positive lead 14couples the positive electrode 11 to the sealing cover 18. The negativelead 15 couples the negative electrode 12 to the negative case 17. Aninsulating gasket 20 is provided to make a pressure-tight joint betweenthe sealing cover 18 and the negative case 17, so that the battery isclosed. Placed between the positive external terminal 16 and the sealingcover 18 is a coil spring 19 which is compressed responsive to theabnormal build-up of a battery's internal pressure to release a gaswithin the battery to outside.

[0043] (Detailed Analysis)

[0044] The hydrogen storage alloy samples A1-A6 and the comparativehydrogen storage alloy sample X were measured for percentages by numberof atoms of elements present therein by using a scanning transmissionelectron microscope and an energy dispersive X-ray analyzer. Thepercentage by number of atoms of an element, as used herein, refers to aratio of the number of atoms of each element to the total number ofatoms of all the elements as detected by the scanning transmissionelectron microscope and the energy dispersive X-ray analyzer, and isgenerally represented by the unit of atomic %.

[0045] A measurement sample was prepared by slicing an alloy particlerepresentative of each hydrogen storage alloy sample. A centrallylocated region of the measurement sample that had a CaCu₅-type crystalstructure and a substantially uniform composition was defined as thebulk region. The region surrounding the bulk region and having a gradedcomposition was defined as the surface region. The percentage by numberof atoms of each element present in the surface region was determined bythe value measured at the intermediate depth thereof, as stated earlier.

[0046] In the manner as described above, the percentages by number ofcobalt and copper atoms present in the surface region and their sum, a,as well as the percentages by number of cobalt and copper atoms presentin the bulk region and their sum, b, were determined to calculate avalue for a/b.

[0047] Specifically for the sample A1, the percentages by number ofcobalt and copper atoms present in the surface region and their sum, a,were determined as being 17.1 atomic %, 2.3 atomic % and 19.4 atomic %,respectively. On the other hand, the percentages by number of cobalt andcopper atoms present in the bulk region and their sum, b, weredetermined as being 14.9 atomic %, 0 atomic % (i.e., no copper atomdetected) and 14.9 atomic %, respectively. The value for a/b wasaccordingly 1.30.

[0048] (Evaluation of Performance Characteristics)

[0049] Each battery was measured for discharge capacity after 500cycles. Each battery was charged at the 0.2 C rate at room temperaturefor 6 hours and then discharged at the 0.2 C rate to 1.0 V. This unitcycle was repeated 500 times. The battery was charged once more tomeasure a 501st cycle discharge capacity (mAh) which was taken as thedischarge capacity after 500 cycles.

[0050] The following procedure was used to evaluate internal pressurecharacteristics of batteries: Each battery was charged at the 1.0 C ratewhile monitoring its internal pressure. When the internal pressurereached 10 kgf/cm², the time (min) was recorded as being indicative ofits internal pressure characteristics.

[0051] The following procedure was used to measure a high-rate dischargecapacity: After activation, each battery was charged at the 0.2 C rateat room temperature for 6 hours and then discharged at the 6.0 C rate to1.0 V to measure a capacity (mAh). The measured capacity (mAh) wasrecorded as the high-rate discharge capacity.

[0052] The measurement results, i.e., the ratio a/b, discharge capacityafter 500 cycles, internal pressure characteristics and high-ratedischarge characteristics, for each of the batteries incorporating thesamples A1-A6 and comparative sample X, are given in Table 1. TABLE 1CoCl₂, CuCl₂, Capacity Internal High-rate Parts Parts (mAh) pressuredischarge Sample by by Ratio after characte- capacity No. weight weighta/b 500 cycles ristics (min) (mAh) A1 0.1 0.1 1.30 760 130 805 A2 0.10.5 1.32 765 135 805 A3 0.1 1.0 1.34 770 135 805 A4 0.1 3.0 1.35 770 135805 A5 0.1 5.0 1.36 775 135 805 A6 0.1 7.0 1.36 740 120 790 X  0 0 1.28720 110 770

[0053] For the samples A1-A6 in accordance with the present invention,respectively prepared via treatment with the aqueous solution ofhydrochloric acid containing the cobalt compound, CoCl₂, and the coppercompound, CuCl₂, the ratio a/b of the sum, a, of percentages by numberof cobalt and copper atoms present in the surface region to the sum, b,of percentages by number of cobalt and copper atoms present in the bulkregion was found to satisfy the relationship a/b≧1.3.

[0054] The comparative sample X gave a value of 1.28 for a/b.

[0055] The batteries incorporating the samples A1-A6 that satisfied therelationship a/b≧1.3 gave higher values for discharge capacity after 500cycles and high-rate discharge capacity compared to the comparativebattery incorporating the comparative sample X. They also exhibitedsuperior internal pressure characteristics.

[0056] Next, the amount of cobalt chloride (CoCl₂) added to the acidtreating solution was varied, by weight, to 0.5%, 1.0%, 3.0%, 5.0% and7.0%, based on the weight of alloy particles to be treated, toinvestigate the effect of cobalt chloride loading on battery performancecharacteristics.

[0057] As indicated in Table 2, sample groups B1-B6, C1-C6, D1-D6, E1-E6and F1-F6 correspond to the cases where cobalt chloride was added in theamount by weight of 0.5%, 1.0%, 3.0%, 5.0% and 7.0%, respectively.

[0058] The measurement results, i.e., compositional ratio a/b for eachsample, and discharge capacity after 500 cycles, internal pressurecharacteristics and high-rate discharge capacity for each battery, aregiven in Table 2. TABLE 2 CoCl₂, CuCl₂, Capacity Internal High-rateParts Parts (mAh) pressure discharge Sample by by Ratio after characte-capacity No. weight weight a/b 500 cycles ristics (min) (mAh) B1 0.5 0.11.33 775 130 810 B2 0.5 0.5 1.34 780 135 810 B3 0.5 1.0 1.35 785 135 815B4 0.5 3.0 1.36 795 135 820 B5 0.5 5.0 1.37 795 135 820 B6 0.5 7.0 1.37745 120 795 C1 1.0 0.1 1.35 795 130 810 C2 1.0 0.5 1.36 795 135 815 C31.0 1.0 1.37 800 135 820 C4 1.0 3.0 1.38 805 140 820 C5 1.0 5.0 1.39 805135 820 C6 1.0 7.0 1.39 755 120 795 D1 3.0 0.1 1.36 800 130 810 D2 3.00.5 1.36 805 135 820 D3 3.0 1.0 1.38 810 135 825 D4 3.0 3.0 1.40 820 140820 D5 3.0 5.0 1.41 820 145 820 D6 3.0 7.0 1.41 755 125 800 E1 5.0 0.11.39 805 130 810 E2 5.0 0.5 1.40 805 130 820 E3 5.0 1.0 1.41 810 135 820E4 5.0 3.0 1.42 815 145 820 E5 5.0 5.0 1.43 810 135 810 E6 5.0 7.0 1.43750 125 790 F1 7.0 0.1 1.39 755 130 800 F2 7.0 0.5 1.40 750 125 795 F37.0 1.0 1.41 750 125 795 F4 7.0 3.0 1.41 745 125 795 F5 7.0 5.0 1.41 740120 795 F6 7.0 7.0 1.41 735 115 790

[0059] As apparent from the results shown in Table 2, the batteriesincorporating samples containing 0.1-5.0% by weight of copper chloride,i.e., samples B1-B5, C1-C5, D1-D5, E1-E5 and F1-F5, show high values fordischarge capacity after 500 cycles, 740 mAh and greater. They also showhigh values for high-rate discharge capacity, 795 mAh and greater. It isthus understood that the cobalt chloride loading is preferably in therange of 0.1-5.0% by weight.

[0060] Based also on the results shown in Table 1, it is understood thatthe preferred loadings of cobalt and copper compounds are both in therange of 0.1-5.0% by weight.

[0061] Although cobalt chloride and copper chloride were used in theExperiment 1 for the cobalt and copper compounds, the similar resultsare obtained with the use of other types of cobalt and copper compounds,such as cobalt hydroxide (Co(OH)₂) and copper hydroxide (Cu(OH)₂).

[0062] Although an aqueous solution of hydrochloric acid was used as theacid treating solution in the producing step of hydrogen storage alloy,i.e., in step 2 of Experiment 1, the similar trend is observed with theuse of nitric acid or phosphoric acid.

[0063] (Experiment 2)

[0064] In this Experiment 2, the effect of a pH of the acid treatingsolution on battery performances was investigated by varying the amountof hydrochloric acid added to the acid treating solution in step 2.Here, cobalt chloride was used for the cobalt compound and copperchloride for the copper compound.

[0065] First, an aqueous solution of hydrochloric acid was preparedwhich contained cobalt chloride and copper chloride in the amountsindicated in Table 2, respectively based on the weight of alloyparticles to be treated, and which was maintained at a pH in the rangeof 0.3-2.5 by controlled addition of hydrochloric acid. The alloyparticles produced in the above Experiment 1 were added to theabove-prepared aqueous hydrochloric acid solution, stirred for 30minutes, suction filtered, washed with water and then dried. Each typeof chloride compound was added to the samples G1-G5 in the amount of0.5% by weight, to the samples H1-H5 in the amount of 1.0% by weight, tothe samples J1-J5 in the amount of 3.0% by weight, and to the samplesK1-K5 in the amount of 5.0% by weight. The procedure of Experiment 1 wasrepeated using these samples to assemble batteries. The measurementresults of discharge capacity after 500 cycles, internal pressurecharacteristics and high-rate discharge capacity for each battery aregiven in Table 3. TABLE 3 CoCl₂, CuCl₂, Capacity Internal High-rate Sam-Parts Parts pH of (mAh) pressure discharge ple by by Treating aftercharacte- capacity No. weight weight solution 500 cycles ristics (min)(mAh) G1 0.5 0.5 0.3 730 115 775 G2 0.5 0.5 0.7 780 135 805 G3 0.5 0.51.0 780 135 810 G4 0.5 0.5 2.0 775 135 805 G5 0.5 0.5 2.5 740 135 780 H11.0 1.0 0.3 750 120 780 H2 1.0 1.0 0.7 800 120 820 H3 1.0 1.0 1.0 800135 820 H4 1.0 1.0 2.0 800 135 820 H5 1.0 1.0 2.5 750 130 780 J1 3.0 3.00.3 750 120 780 J2 3.0 3.0 0.7 820 120 820 J3 3.0 3.0 1.0 820 140 820 J43.0 3.0 2.0 815 140 820 J5 3.0 3.0 2.5 750 140 775 K1 5.0 5.0 0.3 750120 775 K2 5.0 5.0 0.7 810 120 810 K3 5.0 5.0 1.0 810 135 810 K4 5.0 5.02.0 810 130 810 K5 5.0 5.0 2.5 745 120 775

[0066] As apparent from the results shown in Table 3, the batteriesincorporating samples treated with the acid treating solution having apH kept within the range of 0.7-2.0, i.e., samples G2-G4, H2-H4, J2-J4and K1-K4, show high values for discharge capacity after 500 cycles, 780mAh and greater. They also show high values for high-rate dischargecapacity, 805 mAh and greater. These demonstrate that the pH of the acidsolution is preferably kept within the range of 0.7-2.0.

[0067] Although an aqueous solution of hydrochloric acid was used as theacid treating solution in step 2 of Experiment 2, the similar trend isobserved with the use of nitric acid or phosphoric acid.

[0068] Although cobalt chloride and cobalt hydroxide were used inExperiment 2 for the cobalt and copper compounds, the similar resultsare obtained with the use of other types of cobalt and copper compound,such as cobalt hydroxide and copper hydroxide.

[0069] (Experiment 3)

[0070] In this Experiment 3, the effect of inclusion of an organicadditive in the acid treating solution on battery performancecharacteristics was investigated by varying the amount of2,2′-bipyridyl, as a representing organic additive added to the acidtreating solution in step 2. Here, cobalt chloride was used for thecobalt compound and copper chloride for the copper compound.

[0071] First, an aqueous hydrochloric acid solution having a pH of 1.0was prepared which contained cobalt chloride and copper chloride each inthe amount of 1.0% or 3.0% by weight and 2,2′-bipyridyl in the amountindicated in Table 4, respectively based on the weight of alloyparticles to be treated. The alloy particles produced in Experiment 1were immersed in the above-prepared aqueous hydrochloric acid solution,stirred for 30 minutes, suction filtered, washed with water and thendried. As a result, samples L1-L6 and M1-M6 were obtained. The procedureof Experiment 1 was repeated using these samples to assemble batteries.Each battery was measured for discharge capacity after 500 cycles,internal pressure characteristics and high-rate discharge capacity. Themeasurement results are given in Table 4. TABLE 4 CoCl₂, CuCl₂, CapacityInternal High-rate Sam- Parts Parts 2,2′-bi- (mAh) pressure dischargeple by by pyridyl after characte- capacity No. weight weight (ppm) 500cycles ristics (min) (mAh) L1 1.0 1.0 0 800 135 820 L2 1.0 1.0 1.0 805135 820 L3 1.0 1.0 5.0 810 140 825 L4 1.0 1.0 10.0 820 140 830 L5 1.01.0 50.0 810 140 825 L6 1.0 1.0 100.0 800 135 820 M1 3.0 3.0 0 820 140820 M2 3.0 3.0 1.0 825 140 820 M3 3.0 3.0 5.0 830 140 830 M4 3.0 3.010.0 840 145 840 M5 3.0 3.0 50.0 830 140 830 M6 3.0 3.0 100.0 820 140820

[0072] As apparent from the results shown in Table 4, the use of samplestreated with the acid treating solution containing the organic additive,2,2′-bipyridyl, in concentrations of 1.0-50.0 ppm results in theimproved battery performance characteristics. The addition of organicadditive in the amount of 5.0-50.0 ppm is particularly preferred.

[0073] Although 2,2′-bipyridyl was used as a representative organicadditive in Experiment 3, the similar results are obtained with the useof other types of organic additives, such as diethyldithio carbamate,2-mercaptobenzothiazole and metanilic yellow.

[0074] The alloy particles used in the above Experiments were thoseproduced by allowing a mixture of constituent metals to melt in anelectric arc furnace under argon atmosphere and then cool into an ingotand mechanically subdividing the ingot. The similar results are obtainedwith the use of alloy particles produced according to a gas atomizing orroll quenching technique.

[0075] The present invention can sustain the activity of alloy particlesurfaces and improve the electrical conductivity between alloyparticles. An electrode fabricated from such alloy particles, when usedas a negative electrode of a nickel-hydrogen storage battery, canprovide an excellent charge-discharge cycle life performance, preventthe buildup of battery's internal pressure during overcharge and improvehigh-rate discharge characteristics.

1. A method for production of a hydrogen storage alloy, for use inalkaline storage batteries, including: a first step wherein alloyparticles are prepared having a CaCu₅-type crystal structure andrepresented by the compositional formula MmNi_(x)Co_(y)Mn_(z)M_(1−z),wherein M represents at least one element selected from the groupconsisting of aluminum (Al) and copper (Cu), x is a nickel (Ni)stoichiometry and satisfies 3.0≦x≦5.2, y is a cobalt (Co) stoichiometryand satisfies 0≦y≦1.2, z is a manganese (Mn) stoichiometry and satisfies0.1≦z≦0.9, and the sum of x, y and z satisfies 4.4≦x+y+z≦5.4; and asecond step wherein said alloy particles are immersed in an acidtreating solution containing a cobalt compound and a copper compound,each in the amount of 0.1-5.0% by weight based on the weight of alloyparticles, and an organic additive to thereby remove oxide films onalloy particle surfaces and also reductively deposit cobalt and copperso that surface regions are formed at alloy particle surfaces.
 2. Themethod for production of a hydrogen storage alloy as recited in claim 1,wherein said surface region surrounds a bulk region having a CaCu₅-typecrystal structure and a substantially uniform composition and wherein,when the sum of percentages by number of cobalt (Co) atoms and copper(Cu) atoms present in the surface region is given by a and the sum ofpercentages by number of cobalt (Co) atoms and copper (Cu) atoms presentin the bulk region is given by b, the relationship a/b≧1.3 is satisfied.3. The method for production of a hydrogen storage alloy as recited inclaim 1, wherein said organic additive is at least one selected from thegroup consisting of 2,2′-bipyridyl, diethyldithio carbamate,2-mercaptobenzothiazole and metanilic yellow.
 4. The method forproduction of a hydrogen storage alloy as recited in claim 1, whereinsaid organic additive is incorporated in the acid treating solution inthe amount of 5-50 ppm.
 5. The method for production of a hydrogenstorage alloy as recited in claim 1, wherein said acid treating solutionhas a pH of 0.7-2.0.
 6. The method for production of a hydrogen storagealloy as recited in claim 1, wherein, in the first step, said alloyparticles are prepared by a gas atomizing method.
 7. A method forproduction of a hydrogen storage alloy electrode wherein the hydrogenstorage alloy produced according to the method as recited in claim 1 isloaded in an electrically conductive substrate.